Shrinkage performance of several common heat shrinkable films

The production process and principle of heat shrinkable film. Plastics include amorphous plastics and crystalline plastics. In the process of producing the film, the plastic is heated and melted, the force between the macromolecules is weakened, and the macromolecules are arranged in disorder. Even after the film is cooled, as long as the temperature is lower than the softening point, the macromolecules are still in an irregular state. If it is reheated to the high elastic state temperature of the polymer, the film is stretched, and the macromolecular chains will be arranged in a regular orientation along the direction of external force (stretching direction). At this time, when the film is cooled, the orientation of the molecular chain is frozen. When the film is reheated, due to the activity of the molecular segments, the polymer has a tendency to restore its size before stretching (memory function), the stretched oriented film produces stress relaxation, and the oriented film deoriented , The film shrinks and returns to its original size along the original stretching direction. This is the thermal shrinkage of the film. The longitudinal shrinkage rate analysis of heat shrinkable film is based on the temperature and time changes to determine the longitudinal shrinkage length of the film, and the longitudinal shrinkage rate of the film is calculated, see Figure 1. Comparing Figures 1a-c, it can be seen that the longitudinal heat shrinkage rate of several heat shrinkable films in this test increases with the increase of heating temperature or heating time during packaging operation. Among them, the longitudinal heat shrinkage rate of PE film increases the most. The shrinkage rate can range from 45% to 70%, which can be explained from the following two aspects: First, the crystallinity of the PE film is higher than the other three films. Generally speaking, the higher the crystallinity of the film, the tighter the molecular arrangement. , The higher the density, the greater the shrinkage rate; on the other hand, the reason may be that the longitudinal stretching of the PE film is also relatively high, so the thermal shrinkage rate is also relatively large. Comparing Figures 1a-c, it can be seen that PE film has better thermal stability in the range of 150 to 180 ℃. When the PET film is heated for 10 s at a temperature of 100 to 120 ℃, the thermal shrinkage rate is also relatively large, which may also be caused by its relatively high crystallinity; but with the further increase of temperature and heating time, its thermal shrinkage rate changes Not much, it may be caused by the relatively low longitudinal stretch of the PET heat shrinkable film. The longitudinal heat yield of PVC and PVC is not much different, but because the crystallinity of PVC is slightly higher than that of PVC, its longitudinal shrinkage is also slightly higher. Figure 1 Longitudinal thermal shrinkage rate-temperature curve of film. Transverse shrinkage rate analysis of film material. According to temperature and time changes, the transverse shrinkage length of the film is measured, and the transverse shrinkage rate of the film is calculated. Comparing Figures 2a, b and c, it can be seen that the transverse heat shrinkage rate still has a positive dependence on the heating temperature and heating time during packaging operations, and its change with temperature and time is similar to the longitudinal shrinkage, but it may be due to its transverse stretch The degree is different, so the shrinkage rate is not the same. For example, the transverse heat shrinkage rate of PE film is about 30%-40%, while the transverse heat shrinkage rate of PET can reach more than 70%, and the transverse heat shrinkage rate of PVC is 40%-45%. It is believed that the main reason for the above difference in lateral shrinkage is the difference in the degree of lateral stretch of these films. Comparing Figure 1 and Figure 2, the transverse thermal stability temperature range of these films is basically the same as the longitudinal thermal stability range. For example, the transverse thermal stability of PE film is good in the range of 150 ～ 180 ℃; PET is 120 ～ 200 ℃ Good thermal stability. Figure 2 The transverse heat shrinkage rate-temperature curve of the film. Conclusion Based on the above film longitudinal and transverse shrinkage rates, the characteristics of several heat shrinkable films can be compared, see Table 1. PE film has a shorter heating time, more uniform vertical and horizontal shrinkage, and low cost, but the heating temperature range is narrow; PET film has a higher cost and high horizontal shrinkage, which may be more suitable for the process design of horizontal shrinkage; compared to the first two The material, PVC film is moderately costly, and the shrinkage rate is more uniform, but the heating time is slightly longer. PVC is slightly impurity, black, and PVC has better light transmittance. In production, different packaging materials can be selected according to cost and different design schemes, such as: for heat-shrinkable films with printed graphics, it is required to have a balanced heat shrinkage rate in the vertical and horizontal directions; for soft packaging, excessive heat shrinkage It will cause deformation of the object; for cylindrical hard objects, a higher transverse heat shrinkage rate is required to make the film close to the object. And because the actual heat shrinkage rate of heat shrinkable film is often determined by the size of the packaged object, in the actual application of heat shrinkable film, it does not necessarily reach its maximum heat shrinkage rate, and can be adjusted according to actual requirements. Previous: How to use the heat-shrinkable film of Moutai collection dry goods Next: How to choose the greenhouse film, which is better, PO film or PE film?